[media] em28xx: VIDIOC_ENUM_FRAMESIZES: consider the scaler limits when calculating the minimum frame size

Output resolutions <=20% of the input resolution exceed the capabilities of the
scaler.
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>